This invention relates to high energy density electrochemical cells. More specifically, this invention pertains to high energy density rechargeable cells having a novel composition and structural design for the positive electrode to enhance the performance and safety characteristics of the cell during overdischarge and voltage reversal abuses.
In response to an increasing demand for sophistication and miniaturization in energy conversion and storage devices by users in electronics, electro-medical and other industries, many high energy density primary and secondary cells have been developed in recent years. Among these are the ambient temperature nonaqueous electrolyte cells using alkali metals such as lithium or alkaline earth metals such as calcium as anode active materials and insertion compounds such as manganese oxides or soluble oxidizing agents such as sulfur dioxide as cathode active materials. The high energy density permits the miniaturization of the cells without sacrificing performance. On the other hand, cells with high energy densities are susceptible to damage under certain abusive conditions, especially when they are capable of delivering high currents. One such abuse occurs in a multi-cell battery where a cell is overdischarged and driven into voltage reversal by other cells in the circuit. The cause for such an occurrence is that in practice it is difficult to manufacture cells with identical capacities and identical internal impedances. Therefore, it is possible that one cell will exhaust its capacity before the remaining cells in the battery during discharge. Under this condition, the cell with nearly exhausted capacity can be driven to voltage reversal by the remaining cells in the battery. Various electrochemical reactions occur during voltage reversal. In the event that these electrochemical reactions are not controlled, excessive local heating, or more severely, run-away heating will occur leading to cell bulging, venting or rupturing.
Attempts have been made by those who practice in the art to enhance the abuse resistance of high energy density nonaqueous primary cells having an alkali or alkaline earth metal anode. A design feature is taught in U.S. Pat. No. 4,622,277 to prevent cells with the spirally wound cell structure from bulging or venting during voltage reversal abuse. The design feature comprises a first segment of exposed inert metal connected to the cathode and a dendrite target of a second segment of exposed inert metal connected to the anode. The two segments of exposed metal are oriented to face each other but are held in physical isolation by the separator interposed between them. During voltage reversal dendrites grow from the first segment of inert metal to the dendrite target thereby forming a least resistance path between the two electrodes for the current to pass through without generating excessive heat. As a result, the cell is safer and more abuse resistant. It should be noted that U.S. Pat. No. 4,622,277 relates to cells containing "inert" metal cathode current collectors (e.g. aluminum). Inasmuch as lithium can form alloys with aluminum at room temperature, aluminum may not qualify as an inert metal in a cell containing a lithium metal or lithium alloy anode, depending upon the relative capacity of the electrodes. It is true that a lithium cell with an anode coulombic capacity not more than the cathode capacity (a balanced or anode limited cell design) has insufficient lithium to alloy with aluminum hardware at cathode potential. Accordingly, aluminum can be regarded as "inert". On the other hand, aluminum may not be "inert" in a lithium cell containing an excess of negative electrode material, i.e. the coulombic capacity of the negative electrode exceeds that of the positive electrode. Indeed, this is the case in high energy density lithium rechargeable cells which are generally designed to contain an excess of negative electrode material to improve the rechargeability.
In high energy density nonaqueous electrolyte cells the cathode materials are strong oxidizing agents. Therefore, the cathode current collectors must be corrosion resistant and compatible both physically and chemically with the cathode and electrolyte. Corrosion resistant metals such as aluminum, titanium, tantalum and niobium are suitable positive electrode current collector materials. Aluminum is the preferred material due to its low cost and compatibility with a variety of cathodes and electrolytes. Although these metals are corrosion resistant and compatible with respect to the cathode and electrolyte materials, some of them often form alloys or intermetallic compounds with alkali or alkaline earth metals. For example, alloys or intermetallic compounds such as AlLi, Al.sub.3 Mg.sub.2, Al.sub.2 Ca and Al.sub.5 Ba.sub.4 have been reported in the literature and phases diagrams of the Al-Li, Al-Ba, Al-Ca, Al-Mg binary systems can be found in "Moffatt, W.G., The Handbook of Binary Phase Diagrams, 1984 Revision, Genium Publishing Corp, Schenectady, N.Y.". In the event that a cell, wherein the anode capacity is higher than the cathode capacity, is subjected to an overdischarge abuse, the excess of anode material will reach the cathode through the electrolyte to form alloys or intermetallic compounds with the accessible current collector material at the stage of cathode exhaustion. These alloys or intermetallic compounds tend to be grainy and brittle, thus their formation may lead to the destruction of the physical integrity of the current collector.
This problem is severe in the case of a secondary or rechargeable cell wherein the current collector of the positive electrode is likely to be attacked by the negative electrode material repeatedly during multiple discharge/charge cycles and multiple voltage reversals. This may lead to the loss of electrical continuity of the positive electrode, formation of dendritic bridges at unpredictable sites and other unpredictable and unsafe situations.
The possibility of alloy formation between Al (cathode current collector material) and Li in primary Li/SO.sub.2 cells has been postulated (see Levy, S. C. and Crafts, C. C. , The Electrochemical Society Fall Meeting Extended Abstract No. 14, Oct. 11-16, 1981, Denver, Colo.) to explain the shock sensitivity of some Li/SO.sub.2 cells after discharge. One of the design changes suggested by Levy and Crafts to alleviate the shock sensitivity of Li/SO.sub.2 cells was to increase the length of the cathode current collector.
It is important to note, however, that in the high energy density rechargeable cell of this invention, an increase in the length of Al current collector in a rectangular positive electrode beyond the positive electrode material coverage would not improve the safety characteristics during overdischarge and voltage reversal abuses. In other words, the added current collector material which is not physically in direct opposition to the active negative electrode (i.e. not locally available) cannot act as a source for the collector material contributing to the enhancement of current collector integrity during abuse. In the event that the geometric area of the negative electrode is large enough to be directly opposite to not only the positive electrode but also a portion of the bare current collector, the bare and uncovered (by positive electrode material) current collector tends to be preferentially "attacked" by the negative electrode to form alloys or intermetallic compounds during deep discharge cycles, especially at the interface line where the positive electrode material coverage ends and bare collector surface begins. Thus, the loss of electrical continuity may occur in the positive electrode even earlier than in an electrode with no extra length of current collector.